In order to maximize the information content transmitted over a given spectral bandwidth (often measured in bits per Hz of spectral bandwidth), polarization multiplexing (referred to as “Pol-Mux”) is being increasingly used with new transmission formats. The underlying idea is that the spectral density (conveniently measured in units of bits/Hz) can be effectively doubled by employing two orthogonally polarized data-carrying signals sharing the same optical signal bandwidth. Normally, these two orthogonally polarized signals are transmitted with approximately the same intensity, rendering the total resultant light effectively unpolarized as seen from a test and measurement instrument having low electronic detection bandwidth, such as Optical Spectrum Analyzers (OSA).
The Optical Signal-to-Noise Ratio (OSNR) is a direct indicator of the quality of signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE), which is a broadband noise source contributed by the optical amplifiers in the link. In general, the ASE may be considered to be spectrally uniform across the small wavelength range spanning the signal spectral width.
The IEC 61280-2-9 Fiber-optic communication subsystem test procedures—Part 2-9 standards (ed. 1.0 b: 2002) provides a standard method for determining OSNR in Dense Wavelength Division Multiplexing (DWDM) networks. This method is based on the assumption that the interchannel noise level is representative of the noise level at the signal peak position. The method interpolates the power level of the noise outside the signal bandwidth to evaluate the in-band noise in the signal bandwidth. Increased modulation rates, which enlarge the signal bandwidth, and increased channel density, reduce the interchannel width; therefore resulting in severe spectral characteristics requirements for the optical spectrum analyzers used to perform the measurement. The procedures described in the standards are able to cope with these difficulties when the noise level of adjacent peaks is mostly continuous. For example, the standards propose a two-scan procedure to first measure a broad modulated peak with a larger resolution bandwidth to capture the entire signal peak and then determine the noise using a narrow resolution bandwidth to minimize the contributions of the main and adjacent peaks on the interchannel noise level. Alternatively, commercial Optical Spectrum Analyzers (OSA) (such as EXFO's FTB-5240, in versions available before 2007) implement a related procedure by performing an integrated peak calculation and fine noise determination in a single scan.
However, to strictly comply with the standards recommendation, the noise level should be determined at the mid-channel spacing between peaks. In the case where noise is spectrally filtered outside the optical signal bandwidth, for instance, after passing through multiplexers or demultiplexers—such as Reconfigurable Optical Add Drop Multiplexers (ROADM)—the mid-spacing noise level is no longer representative of the in-band noise level, which is the relevant parameter for the OSNR determination. The interpolation of the interchannel noise level then becomes unreliable. This can be mitigated by relying on a very sharp spectral response of the OSA filter and adaptive processing to determine the noise level at the shoulders where the noise meets the base of a signal profile within the channel bandwidth. However, increased modulation rates combined with narrow filtering of multiplexers and demultiplexers is making it increasingly difficult to achieve a reliable measurement of the noise level within the channel bandwidth.
Alternative in-band OSNR measurement methods have been developed for DWDM network applications. Such methods include the active polarization-nulling method (see J. H. Lee et al., “OSNR Monitoring Technique Using Polarization-Nulling Method”, IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001) and the Passive Polarization-Induced Discrimination (PPID) approach (see International Patent Application Publication WO 2008/122123 A1 to Gariépy et al., commonly owned by The Applicant). However, such methods are based on the assumption that the signal is generally highly polarized, an assumption that is not valid in the case of polarization-multiplexed signals.
For the case of most polarization-multiplexed signals, the “signal”, as detected on a photodiode having low bandwidth electronics for instance, appears unpolarized, and hence, these above-mentioned in-band OSNR measurement methods cannot be used to reliably provide the OSNR measurement.
In order to measure the noise level or the OSNR on polarization-multiplexed signals, system manufacturers and operators currently have to resort to turning off the signal at the transmitter in order to measure the noise level and thereby determine the OSNR. A first limitation of this method is that it requires making certain assumptions about the noise variations that occur upon turning off the signal for which the OSNR needs to be measured. The OSNR measurement uncertainty depends, for example, on the number of channels on the link sharing the same amplified paths. In cases where the measurement is to be carried out on a system that is in operation, such a method involving turning off the signal has the important practical limitation that it implies a service interruption for the channel of interest and possible disruption of the other channels on the system.
There is therefore a need for a method to measure in-band noise parameters such as the OSNR on polarization-multiplexed signals or any other unpolarized signals, without service interruption.